Apparatus for making x-ray images

ABSTRACT

An apparatus for making X-ray images includes an X-ray source which is controlled by an X-ray source control. A luminescent screen provides a visible image based upon X-rays transmitted by the X-ray source. A CCD camera receives and stores the visible image. Imaging optics including a mirror reflect the visible image to the camera without splitting the visible image into parts. A photosensor is in optical communication with the luminescent screen and is separate from the CCD camera. The photosensor is positioned such that an illumination power of the luminescent screen falls on the photosensor practically from within an area of an entire portion of the visible image. A computer and control connectable to the X-ray source control for adjusting exposure of the apparatus is connected to the photosensor.

FIELD OF THE INVENTION

The Invention relates to an apparatus for making X-ray images.

BACKGROUND OF THE INVENTION

In clinical diagnostics, the quality of X-ray images is a very importantfactor. Furthermore, archiving and further processing of the images in ahospital environment imposes many special demands on the apparatus bywhich the X-ray images are made. One factor which increases theusability of the apparatus its small size and light weight.

Conventional apparatus, for example used for X-ray pneumography,comprises a plate on which a visible image is formed due to X-raysincident on the plate, and the film placed on the plate is exposed. Theplate and the film are disposed in a cassette which is removed aftereach exposure. An apparatus is also known in which a latent image isformed on a drum with a selenium surface. Imaging comprises charging thedrum by rotating the drum, image formation, and scanning of it forstorage again by rotating the drum. The apparatus contains moving partsand is heavy.

Conventional X-ray apparatus has a measuring chamber in front of thephotographic plate which measures the X-ray intensity incident on theplate and sends out a message to an automatic exposure control unit inconnection with the X-ray generator after a specific exposure isattained. In this way, one tries to achieve a correct exposure at eachpicture taking.

An apparatus is known from U.S. Pat. No. 5,187,730 for an X-raydiagnostic system and method of picking up X-ray picture. The image isformed by an image intensifier which converts the X-rays intofluorescent light. There is also an optical system having a lens meansfor converting the fluorescent light and a beam splatter means forsplitting the fluorescent light into a plurality of optical paths. Thebeam splitter is a partly transparent mirror so that a part of thefluorescent light is led to a photodetector, such as a photodiode. Inthe diagnostic system according to the above U.S. patent, there is arisk that part of the X-rays penetrate the reflecting mirror and theX-rays may cause damage to the CCD. Only a part of the fluorescent lightis reflected to the photosensor so the quantity-of-light controller mayuse incorrect values in controlling the X-ray generator.

An X-ray imaging system including brightness control is also known fromEuropean Patent Application 547679. In this application, the image fromthe X-ray image intensifier is conducted through prisms. Part of thebeam is reflected to a photodiode for automatic dose control or exposuretiming. Also, in this apparatus, there is the disadvantage that someX-rays may penetrate through the prisms to the CCD sensor which may bedamaged. Another disadvantage is that the photosensitive sensor seesonly part of the light beam from the image intensifier.

An apparatus is also known from the European patent 264218 for making animage of an object by X-rays in which the image formed on a phosphorscreen by the X-rays is stored by means of a CCD camera after which theimage can be processed digitally. Exposure time is controlled by ashutter and the CCD is operated in slow scan mode which involves coolingdown to at least -40° C. to decrease noise due to slow exposure of thepicture and/or the read-out. Exposure times with this apparatus aregenerally minutes or even hours. The apparatus is adapted for X-rayinspection of objects, but it does not take into account needs ofclinical X-raying.

SUMMARY OF THE INVENTION

The purpose of the invention is to obtain a new kind of apparatus formaking X-ray images which is particularly suitable for X-raying patientsin clinical examinations like for X-raying of lungs and bones. Toaccomplish this objective, the apparatus of this invention is providedwith a photosensor which is separate from the CCD camera and is inoptical communication with the image formed on the luminescent screenwhich sends out light of visible wavelength. The photosensor ispositioned so that the illumination power of the luminescent screenfalls on it practically within the area of the entire image. Thus, theintensity of the image formed can be directly measured allowing thisinformation to be used for adjusting correct efficiency of exposure,time or quantity of exposure by connecting the sensor to the X-raygenerator which controls the X-ray source. At the same time, a scale canbe automatically set in a correct position in terms of the imageprocessing and digitizing. The imaging optics also includes a mirror forreflecting the image on the luminescent screen to the CCD camera. Theoptical reflecting surface formed by the mirror also prevents X-rayswhich may have got through from hitting the sensitive imagingelectronics.

As to the other advantageous embodiments of the invention, reference ismade to the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the followingreferring to the enclosed drawings in which

FIG. 1 shows the apparatus according to the invention as a section,

FIG. 2 shows a second embodiment of the invention as a section,

FIG. 3 shows how the dynamical range is extended in the apparatusaccording to the invention, and

FIG. 4 shows the location of the apparatus in a larger system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the apparatus according to the invention as a section. Thefigure shows the grid 1 of the apparatus arranged to diminish theinjurious effect of the secondary radiation from the patient. The X-raysthat have passed through the patient are incident on the luminescentscreen 2 forming thereon a visual image which sends out radiation atvisible wavelength. The luminescent screen 2 may be a known phosphorscreen which gets the necessary energy to generate the image from theX-ray radiation; in other words, no amplification is performed by meansof any other externally supplied energy although it is sometimes alsocalled an intensifier screen. The beam cone of the image to be stored ismarked in the drawing with dashed lines and the image is received with a2-dimensional charge-coupled device image transducer (CCD) which is partof the CCD camera 3 which also comprises conventional imaging optics 3a.An interface 4 leads from the camera to the electronic unit 5 of theapparatus which performs image processing, e.g. digitizing, andtransmission of control signals along the communication line 15 furtherto various auxiliary devices 16.

The apparatus is provided with a photosensor 6 which may be any knownsensor measuring intensity of visible light. The sensor is connectedwith a data transfer line 7 to said electronic unit 5 which is providedwith computing and control means for processing the data supplied by thesensor. From this part leads also a data transfer line 8 leads to theX-ray generator (RTG) which controls the X-ray source.

FIG. 1 shows how the cone of the light beams to be imaged is turned bythe mirror 9 to the camera 3 located at the lower part of the apparatus.The photosensor 6 is located beside the light cone of the light beamswhich accomplishes storage of the image, for example in the manner shownin the figure opposite to the mirror 9 beside the CCD camera. Thephotosensor 6 may also be positioned in the apparatus so that itdirectly "sees" the image on the luminescent screen 2; i.e., it is thenopposite to the luminescent screen 2 beside the mirror 9. FIG. 2 showsthe alternative wherein the mirror 9 reflects the cone incident from theluminescent screen 2 to an additional mirror 10 located oppositethereto, arranged below the lower part of the luminescent screen, andfinally reflects the cone to the CCD camera 3 at the lower part of theapparatus. In this case, the apparatus will have a more compactstructure when the above components are placed into the frame housing11. The figure further shows how the sensor 6 may also, in thisalternative, be located opposite mirror 9 and be placed beside mirror10.

Dashed lines indicate the cone of those light beams which leave theluminescent screen 2 at a different angle to the sensor than the cone ofthe light beams which go to the actual image storage. Thus, the sensor 6is not in the way of the light beams, and no prisms or the like areneeded in the apparatus to split the light for storing and measuring theimage.

In the present invention, the mirror (or mirrors) make it possible toprevent X-rays which may have gotten through from hitting the sensitiveelectronics because the optically reflecting surface accomplished by themirror directs only the beams of visible light into the imagingelectronics. The frame housing 11 may be arranged to be movable along avertical guide 12 to the right height. A correct dosage is automaticallyaccomplished by the photosensor 6 because its measuring signal istransmitted to the computing and control means which control the X-raysource to obtain a correct exposure. In practice, this may take place sothat as a predetermined quantity of exposure is reached, a signal istransmitted to the control means of the X-ray source which switches offthe X-ray generator. Exposure times in clinical X-raying may in thiscase be about 10 ms. At the same time, a scale is automaticallyestablished in a correct range from the point of view of imageprocessing. No mechanical shutters are needed in the optical pathbetween the luminescent screen 2 and the 2-dimensional CCD imagetransducer because, between exposures, the image transducer ispractically in darkness inside the frame housing and it can becalibrated by measuring the output signal at this situation.

FIG. 3 shows a typical characteristic curve of a CCD element whichrepresents the signal strength of the element as a function of dosage.Thus, the question is whether the image is obtained from a single imagepoint or pixel. At high dosage, the effect of the anti-blooming actionof the CCD camera manifests itself by preventing the electrons fromescaping to nearby elements and "smearing" of the image at high dosage.In the apparatus, the effect of this so-called saturation region S tothe image is eliminated so that the dynamic range is extended inaccordance with the straight portion of the graph in the linear range Lso that when the obtained signals are in the saturation region S, theyare numerically corrected to correspond to actual values. In this case,high accuracy is obtained in the image processing in regions of highdosage, like in pneumography in those areas of the image obtainedparticularly from the patient's peripheral areas, the corresponding CCDelements of which receive dosage above normal. The correction inquestion can be performed in the electronic unit 5 of the apparatus by asuitable program before the image data is sent out for furtherprocessing.

FIG. 4 finally shows the apparatus in the environment in which thepicture is taken. The apparatus comprises the frame housing 11 and theelectronic unit 5 which communicates with the control means 14 of theX-ray source 13, i.e., the X-ray generator RTG. The system furthercomprises auxiliary devices 16 for storing the image data, displayingthem on the screen or outputting them, or transmitting the image datafurther.

The invention utilizes a high-resolution CCD camera 3 with several imagepoints or pixels. This kind of camera is temperature stabilized, butbecause it operates as a quick camera (flash mode), with exposure timestypically below 100 ms, it needs no cooling to improve thesignal-to-noise ratio. The apparatus suits well for use in connectionwith conventional X-ray instruments which require no changes due to theapparatus.

In connection with the present invention it is advantageous to employ aCCD camera in which the CCD image transducer is as sensitive as possibleso that in the X-raying process the patient is exposed to a minimumamount of X-ray radiation. One such sensitive CCD image transducer is athinned CCD image transducer used in for example astronomical imaginginstruments. In an image transducer of this type, the rear surface ofthe of the COD image transducer is thinned so that light beams can bedirected into the image transducer from the rear side, whereas inconventional CCD image transducers the light beams are directed from thefront side of the image transducer. Sensitivity of the CCD imagetransducer may be further increased by adding a phosphorescent layer,most advantageously to the front side of the CCD image transducer. TheCCD image transducers described above have sensitivities even 2-3 timeshigher as compared to conventional CCD image transducers.

We claim:
 1. An apparatus for making X-ray images, comprising:an X-raysource controlled by an X-ray source control means; a luminescent screenfor providing a visible image based upon X-rays transmitted by saidX-ray source; a CCD camera for receiving and storing said visible image;imaging optics including a mirror for reflecting said visible image tothe camera without splitting said visible image into parts; aphotosensor in optical communication with said luminescent screen andseparate from said CCD camera, said photosensor being positioned suchthat an illumination power of said luminescent screen falls on saidphotosensor practically from within an area of an entire portion of saidvisible image; and computing and control means connected to saidphotosensor, said computing and control means being connectable to saidX-ray source control means for adjusting exposure of said apparatus. 2.Apparatus according to claim 1, wherein the imaging optics and thephotosensor are positioned apart so that a first cone collected by theimaging optics and a second cone collected by the photosensor leave theluminescent screen at an angle to each other.
 3. Apparatus according toclaim 2, wherein the photosensor is positioned opposite the mirror sothat the illumination power reflected by the mirror is transmitteddirectly thereto.
 4. Apparatus according to claim 3, wherein thephotosensor is positioned beside the CCD camera to receive lightreflected by the mirror.
 5. Apparatus according to claim 3, furthercomprising an additional mirror for reflecting the visible imagereflected by the mirror to the CCD camera, the photosensor beingdisposed beside the additional mirror.
 6. Apparatus according to claim1, further comprising an electronic unit including a program which isarranged to correct at high quantities of exposure a signal obtainedfrom a luminous point in a saturation range to correspond to a dependentof signals in a linear range on a quantity of exposure.
 7. Apparatusaccording to claim 1, wherein the CCD camera comprises a thinnedCCD-image transducer.
 8. The apparatus according to claim 1, whereinsaid luminescent screen is a phosphor screen.